class ConcertTicket(object):

    SECTIONS = {'lower', 'premier', 'mezzanine', 'floor'}

    def __init__(self, price, section):

        self._price = price

        self._section = section

    @property

    def price(self):

        return self._price

    @property

    def section(self):

        return self._section

    @section.setter

    def section(self, section):

        if section not in self.SECTIONS:

            raise ValueError('Invalid section "{}"'.format(section))

        self._section = section

if __name__ == '__main__':

    import doctest

    doctest.testmod()